Direct Current Power Combiner

ABSTRACT

A circuit for combining direct current (DC) power including multiple direct current (DC) voltage inputs; multiple inductive elements. The inductive elements are adapted for operatively connecting respectively to the DC voltage inputs. Multiple switches connect respectively with the inductive elements. A controller is configured to switch the switches periodically at a frequency sufficiently high so that direct currents flowing through the inductive elements are substantially zero. A direct current voltage output is connected across one of the DC voltage inputs and a common reference to both the inputs and the output.

CROSS REFERENCE TO RELATED APPLICATIONS

The present applications benefits from U.S. application 61/050268 filedon May 5, 2008 by the same inventor.

FIELD AND BACKGROUND

1. Field

The present invention relates to energy conversion and specifically tocircuitry which combines multiple voltage inputs from serially connecteddirect current sources into a combined output.

2. Description of Related Art

Sunlight includes a spectrum of electromagnetic radiation emitted by theSun onto the surface of the Earth. On the Earth, sunlight is filteredthrough the atmosphere, and the solar irradiance (Watts/metersquare/nanometer W/m²/nm) is obvious as daylight when the Sun is abovethe horizon. The Earth receives a total solar irradiance determined byits cross section (π·R_(E) ₂ , R_(E)=radius of the earth), but as theEarth rotates the solar energy is distributed across the entire surfacearea (4·π·R_(E) ₂ ). The solar constant is the amount of incoming solarelectromagnetic irradiance per unit area, measured on the outer surfaceof Earth's atmosphere in a plane perpendicular to the solar rays. Thesolar constant is measured by satellite to be roughly 1366 watts persquare meter (W/m²) or 1.366 W/m²/nm. Hence the average incoming solarirradiance, taking into account the angle at which the rays strike andthat at any one moment half the planet does not receive any solarirradiance, is one-fourth the solar constant (approximately 0.342W/m²/nm). At any given moment, the amount of solar irradiance receivedat a location on the Earth's surface depends on the state of theatmosphere and the location's latitude.

The performance of a photovoltaic cell depends on the state of theatmosphere, the latitude and the orientation of the photovoltaic celltowards the Sun and on the electrical characteristics of thephotovoltaic cell.

FIG. 1 shows schematically a graph of a solar irradiance 100 versuswavelength. Irradiance 100 is distributed around a peak wavelength atabout 550 nanometers. FIG. 1 also shows schematically an absorptionspectrum 102 of a typical solar photovoltaic (PV) cell with a givenband-gap which allows only a portion of the solar irradiance to beconverted into electrical power. The finite characteristic of theband-gap of the photovoltaic cell causes a substantial part of the sun'senergy to remain unutilized. In order to improve photovoltaicefficiency, multiple junction cells have been designed which includemultiple pn junctions. Solar irradiance not absorbed, because its energyis less than the band gap is transmitted to the next junction(s) with asmaller band gap and the transmitted radiation is preferentiallyabsorbed and converted into electrical energy.

FIG. 2 shows the graph of solar irradiance 100 versus wavelength andthree absorption spectra 202, 204 and 206 respectively of threephotovoltaic junctions used in a single multi junction cell designed toabsorb different parts of the solar spectrum. The first photovoltaicjunction having the largest band gap has an absorption spectrum 206, thesecond photovoltaic junction has an absorption spectrum 204, and thethird photovoltaic junction which has the smallest band gap has anabsorption spectrum 202. Combining the three pn junctions ofphotovoltaic junctions into a single multi junction 30 cell increasesthe efficiency, theoretically to about 60% and practically today toabove 40%.

FIG. 3 illustrates multiple multi junction cells 30 connected in series.Each multi-junction cell 30 has three serially connected photovoltaicjunctions 300, 302, and 304 which operate with three absorption spectra206, 204 and 202 respectively. Multiple multi junction cells 30connected in series form a multi-spectral photovoltaic panel 3000 withoutput terminals 310 and 308.

FIG. 4 illustrates characteristic current-voltage curves of a singlephotovoltaic junction cell at different illumination levels. Curve 400shows the maximum power point (MPP) for low light levels, curve 402 showthe maximum power point MPP for higher light levels, and curve 404 showsthe maximum power point MPP yet higher light levels assuming a constanttemperature of the cell. As can be seen, at the different light levelsthe maximum power point is achieved at nearly identical voltages, but atdifferent currents depending on the incident solar irradiance.

Reference is now made to conventional art in FIGS. 5a and 5b which showsa typical photovoltaic installation 50 operating in dark or partiallyshaded conditions and bright mode respectively. Bypass diodes 500 a-500c are connected in parallel across photovoltaic panels 502 a-502 crespectively for instance according to IEC61730-2 solar safety standards(sec. 10.18). Photovoltaic panels 502 a-502 c are connected in series toform a serial string of photovoltaic panels. Referring to FIG. 5a ,bypass diode 500 a provides a path 510 around photovoltaic panel 502 aduring dark or partially shaded conditions. Current path 510 allowscurrent to flow through bypass diode 500 a in the forward mode,preventing common thermal failures in photovoltaic panel 502 a like cellbreakdown or hot spots. During forward mode, bypass diode 500 apreferably has low forward resistance to reduce the wasted power. FIG.5b refers to normal operation or bright mode, forward current 512 willflow through photovoltaic panels 502 a-502 c while bypass diodes 500a-500 c will operate in the reverse blocking mode. In reverse blockingmode, it is important that bypass diodes 500 a-500 c have the lowesthigh temperature reverse leakage current (I_(R)) to achieve the highestpower generation efficiency for each photovoltaic panel 502 a-502 c.

BRIEF SUMMARY

According to the present invention there is provided a circuit includingmultiple direct current (DC) voltage inputs which including one or moreshared terminals. A primary transformer winding includes a high voltageend and a low voltage end. The primary transformer winding has a tap ortaps operatively connected to the shared terminals through a firstswitch. A secondary transformer winding includes a high voltage end anda low voltage end. The secondary transformer winding iselectromagnetically coupled to the primary transformer winding. Thesecondary transformer winding has one or more taps operatively connectedto the shared terminal(s) through a second switch. A direct currentvoltage output terminal connects the high voltage ends of the primaryand secondary transformer windings. A low voltage direct current outputterminal operatively connecting said low voltage ends of said primaryand secondary transformer windings.

Diodes are typically connected in parallel with the first and secondswitches or the diodes are integrated with a transistor in a singlepackage. The switches may be metal oxide semiconductor field effecttransistor (MOSFET), junction field effect transistor (JFET), insulatedgate field effect transistor (IGFET), n-channel field effect transistor,p-channel field effect transistor, silicon controlled rectifier (SCR)and/or bipolar junction transistor (BJT). A third switch optionallyconnects the low voltage end of the primary transformer winding to acommon terminal; and a fourth switch optionally connects the low voltageend of the secondary transformer winding to the common terminal. Diodesare typically connected in parallel with the third switch and the fourthswitch. Bypass diodes are operatively connected across the DC voltageinputs. Photovoltaic cells are optionally connected to the DC voltageinputs. The photovoltaic cells may be optimized for maximal opticalabsorption of different respective portions of the electromagneticspectrum. The direct current voltage output terminal may be connected toa DC to DC converter.

According to the present invention there is provided a circuit includingmultiple direct current (DC) voltage inputs; multiple transformersincluding primary windings and secondary windings; multiple firstswitches connected respectively in series with the primary windings intoa multiple of switched primary windings; and multiple second switchesconnected respectively in series with the secondary windings intomultiple switched secondary windings. The switched secondary windingsare parallel connected respectively with the switched primary windingsby the DC voltage inputs. The switched secondary windings are adaptedfor connecting to a combined direct current power output combining theDC voltage inputs. The first and second switches are: metal oxidesemiconductor field effect transistor (MOSFET), junction field effecttransistor (JFET), insulated gate field effect transistor (IGFET),n-channel field effect transistor, p-channel field effect transistor,silicon controlled rectifier (SCR) and/or bipolar junction transistor(BJT).

According to the present invention there is provided a circuit forcombining direct current (DC) power including multiple direct current(DC) voltage inputs; multiple tapped coils including respectivelyprimary ends, secondary ends and taps. The taps are adapted forconnecting individually to the DC voltage inputs. The first switchesconnect respectively in series with the tapped coils at the primary endsof the coils. The second switches connect respectively in series withthe coils at the secondary ends of the coils. The taps serially connectrespectively the first and second switches. A combined direct currentpower output is adapted for connecting between the tap of highestvoltage and a reference to both the inputs and the output.

According to the present invention there is provided a circuit forcombining direct current (DC) power including multiple direct current(DC) voltage inputs; multiple inductive elements. The inductive elementsare adapted for operatively connecting respectively to the DC voltageinputs. Multiple switches connect respectively with the inductiveelements. A controller is configured to switch the switchesperiodically. A direct current voltage output is connected across one ofthe DC voltage inputs and a reference to both the inputs and the output.

According to the present invention there is provided a method forcombining direct current (DC) power. Multiple direct current (DC)voltage inputs are connected to respective inductive elements. Multipleswitches are connected respectively with the inductive elements. Theswitches are switched periodically.

A direct current voltage output is combined by connecting across one ofthe DC voltage inputs and a reference common to both the DC voltageinputs and the direct current voltage output.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a graph illustrating typical spectra of solar irradiance andsolar absorption of a single photovoltaic junction, according toconventional art. .

FIG. 2 is a graph illustrating three different absorption spectra ofthree stacked photovoltaic junctions of a multi junction photovoltaiccell, according to conventional art. .

FIG. 3 illustrates serially connected multi junction cells, according toconventional art.

FIG. 4 illustrates a current-voltage (TV) characteristic curve(arbitrary units) of a photovoltaic cell at three different illuminationlevels, according to conventional art.

FIGS. 5a and 5b illustrates a typical photovoltaic installationoperating in during dark or partially shaded conditions and bright moderespectively, according to conventional art.

FIG. 6 illustrates a block diagram of photovoltaic installation with apower combiner according to an embodiment of the present invention.

FIG. 7 illustrates a power combiner circuit, according to an embodimentof the present invention.

FIG. 8 illustrates a power combiner circuit, according to anotherembodiment of the present invention.

FIG. 9 illustrates a photovoltaic system including multiple powercombiners, according to an exemplary embodiment of the presentinvention.

FIG. 10 illustrates a flow diagram of a method, according to anembodiment of the present invention.

The foregoing and/or other aspects will become apparent from thefollowing detailed description when considered in conjunction with theaccompanying drawing figures.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The embodiments are described below to explain the presentinvention by referring to the figures.

By way of introduction, different embodiments of the present inventionare directed toward compensating for current variations in multiplejunctions cells or in serially connected photovoltaic cells and/orpanels such as during partial shading while maximizing power gain, byavoiding the loss of power from one or more photovoltaic cells and/orpanels shorted by the cells and/or panels respective bypass diode.

Reference is now made back to FIG. 3, which illustrates conventionallymultiple multi-junction cells 30 connected in series, each with multipleserially connected photovoltaic junctions 300, 302, and 304. It is wellknown that the spectrum of solar irradiance on the Earth's surface isnot a constant but varies according to many variables such as season,geographic location, time of day, altitude, atmospheric conditions andpollution. Hence, it becomes apparent that photovoltaic junctions 300,302, and 304 sensitive to different spectrum bands may be absorbing adifferent amount of light depending on season, geographic location, timeof day, altitude, atmospheric conditions and pollution. Sincephotovoltaic junctions 300, 302, and 304 are connected in series, thesame current flows through all of the junctions. Thus, the best powerpoint of serially connected photovoltaic junctions 300, 302, and 304maximizes the overall power from photovoltaic junctions 300, 302, and304, while each junction is typically producing a less than optimalamount of electrical power. On the other hand, a parallel connection ofphotovoltaic junctions and/or multi junction cells, while allowing abetter maximum power control for all photovoltaic junctions ormulti-junction cells suffers among other possible power losses from anincrease of ohmic power loss of the system since ohmic power loss isproportional to the square of the current. Furthermore, a parallelelectrical connection of stacked pn junctions in a multi-junction cellis not particularly practical since multi junction cells are typicallystacked in a single production process and since the MPP voltage of eachof these stacked pn junctions is different; the bandgap voltage for eachpn junction is different.

The present invention in different embodiments may be applied tomultiple photovoltaic cells and/or multi-junction photovoltaic cellsconnected in various series and parallel configurations with powerconverters/combiners to form a photovoltaic panel. Multiple series andparallel configurations of the photovoltaic panel and substrings withina panel with multiple power converters/combiners are used to form aphotovoltaic installation. The present invention in further embodimentsmay be applied to other direct current power sources includingbatteries, fuel cells and direct current generators.

Embodiments of the present invention may be implemented by one skilledin the electronics arts using different inductive circuit elements suchas transformers, auto-transformers, tapped coils, and/or multiple coilsconnected in serial and/or in parallel and these devices may beconnected equivalently to construct the different embodiments of thepresent invention.

The terms “common”, “common terminal, “common reference” are used hereininterchangeably referring to a reference common to both inputs and theoutput in the context of embodiments of the present invention.Typically, “common terminal” is ground, but the whole circuit may alsobe ungrounded. References to common terminal as ground are onlyillustrative and made for the reader's convenience.

Reference is now made to FIG. 6 which illustrates a block diagram ofphotovoltaic installation 600 with a power combiner 604 according to anembodiment of the present invention. A photovoltaic panel 60 has threephotovoltaic cells 606 a-606 c connected in series. Photovoltaic cells606 a-606 c are preferably multi-junction photovoltaic cells,photovoltaic cells or other direct current sources. An anode and cathodeof a bypass diode D₁ connects across in parallel with photovoltaic cell606 c at node F and node A respectively. An anode and cathode of abypass diode D₂ connects across in parallel with photovoltaic cell 606 bat node A and node B respectively. An anode and cathode of a bypassdiode D₃ connects across in parallel with photovoltaic cell 606 a atnode B and node C respectively. Voltages V₁, V₂ and V₃ are the voltageoutputs of photovoltaic cells 606 c, 606 b and 606 a respectively.Voltages V₁, V₂ and V₃ are applied to three voltage inputs of powercombiner 604 as between nodes C & B, B & A and nodes A & F respectively.Power combiner 604 has a single output voltage V_(out).

Reference is now made to FIG. 7 which illustrates, according to anembodiment of the present invention, circuit details of DC powercombiner 604. Three voltages V₁, V₂ and V₃ are input to power combiner604 between nodes A and F, nodes B and A and nodes C and B respectively.Node B is on a “shared input terminal” of V₂ and V₃. Similarly, node Ais on a “shared input terminal” of V₁ and V₂. One end of inductor L₁connects to node C, the other end of inductor L₁ connects to one end ofinductor L₃ to form node W. The other end of inductor L₃ connects to oneend of inductor L₅ to form node X. The other end on inductor L₅ connectsto the drain of MOSFET G₁ and the source of G₁ connects to node F(ground). One end of inductor L₂ connects to node C, the other end ofinductor L₂ connects to one end of inductor L₄ to form node D. The otherend of inductor L₄ connects to one end of inductor L₆ to form node E.The other end on inductor L₅ connects to the drain of MOSFET G₂ and thesource of MOSFET G₂ connects node F (ground). The drain of MOSFET G₅ isconnected to node W, the source of MOSFET G₅ connects to the source ofMOSFET G₆. The drain of MOSFET G₆ connects to node D. The drain ofMOSFET G₄ is connected to node X, the source of MOSFET G₄ connects tothe source of MOSFET G₃. The drain of MOSFET G₃ connects to node E. Theoutput voltage V_(out) of power combiner 604 is derived between nodes Cand F (ground). A transformer core 601 is used to electromagneticallycouple all inductors L₅, L₆, L₃, L₄,L₁ and L₂. The winding polarity ofL₅, L₃ and L₁ is preferably opposite of the winding polarity of L₆, L₄and L₂. The two inductors within each of the inductor pairs L₅-L₆, L₃-L₄and L₁-L₂ typically have the same number of winding turns, althoughthere can be a different number of turns to each of the inductor pairs(eg.L1 and L2, L3 and L4 and L5 and L6), to adjust the typical relativeMPP voltage of each of the input voltages. Each of the three voltagesV₁, V₂ and V₃ are applied across each of inductors L₅, L₃ and L₁respectively with for instance a 50% duty cycle when switches G1, G4 andG5 are closed and switches G2, G3 and G6 are opened. Each of the threevoltages V₁, V₂ and V₃ are applied across each of the inductors L₆, L₄and L₂ respectively with typically a 50% duty cycle when switches G1, G4and G5 are opened and switches G2, G3 and G6 are closed, thus completinga full switching cycle. The output voltage (V_(OUT)) of power combiner604 is the sum of the input voltages V₁, V₂ and V₃. The input voltagesV₁, V₂ and V₃ of power combiner 604 are forced by power combiner 604 tohave the same ratio as the winding ratio of their inductor pair (L₅,L₆), (L₃, L₄) and (L₁, L₂) respectively; a result of applying controlpulses to switches G₁-G₆ for instance with a 50% duty cycle. SwitchesG₁-G₆ are optionally metal oxide semiconductor field-effect transistors(MOSFET). Alternatively the switches can, in different embodiments ofthe invention, be a silicon controlled rectifier (SCR), insulated gatebipolar junction transistor (IGBT), bipolar junction transistor (BJT),field effect transistor (FET), junction field effect transistor (JFET),switching diode, mechanically operated single pole double pole switch(SPDT), SPDT electrical relay, SPDT reed relay, SPDT solid state relay,insulated gate field effect transistor (IGFET), DIAC, and TRIAC.

Reference is now made to FIG. 8 which illustrates, according to anotherembodiment of the present invention, an alternative circuit of DC powercombiner 604. Three voltages V₁, V₂ and V₃ are input to power combiner604 between nodes A & F, B & A and nodes C & B respectively. One end ofinductor L₁ connects to node C, the other end of inductor L₁ connects tothe drain of MOSFET G₁ the source of G₁ connects to node B. One end ofinductor L₃ connects to node B, the other end of inductor L₃ connects tothe drain of MOSFET G₃, the source of G₃ connects to node A. One end ofinductor L₅ connects to node A, the other end of inductor L₅ connects tothe drain of MOSFET G₅, the source of G₅ connects to node F (ground).One end of inductor L₂ connects to node C, the other end of inductor L₂connects to the drain of MOSFET G₂, the source of G₂ connects to node B.One end of inductor L₄ connects to node B, the other end of inductor L₄connects to the drain of MOSFET G₄, the source of G₄ connects to node A.One end of inductor L₆ connects to node A, the other end of inductor L₆connects to the drain of MOSFET G₆, the source of G₆ connects to node F(ground). The output voltage V_(out) of power combiner 604 is derivedbetween nodes C and F (ground). A transformer core 601 is used toelectromagnetically couple all inductors L₅, L₆, L₃, L₄, L₁ and L₂. Thewinding polarity of L₅, L₃ and L₁ is preferably opposite of the windingpolarity of L₆, L₄ and L₂ respectively. The two inductors within each ofthe inductor pairs (L₅ and L₆), (L₃ and L₄) and (L₁ and L₂) preferablyhave the same number of winding turns, although there can be a differentnumber of turns to each of the inductor pairs, so as to adjust thetypical relative MPP voltage of each of the input voltages.

Reference is now made to FIG. 9 which illustrates a photovoltaic system90 including multiple power combiners 604, according to an exemplaryembodiment of the present invention. Photovoltaic system 90 has multipleseries strings 902 connected in parallel to the input of DC to ACconverter 900. Series strings 902 have photovoltaic cells 904 a-904 cwhich are for instance multi-junction photovoltaic cells which havethree voltage outputs V₁, V₂ and V₃ with three bypass diodes connectedacross each voltage output of photovoltaic cells 904 a-904 c. Connectedto each photovoltaic cells 904 a-904 c is a three voltage input powercombiner 604. Power combiner 604 has a single voltage output (V_(out))which is applied across the input of DC to DC converters 92 a-92 c. Theoutputs of DC to DC converters 92 a-92 c are connected in series to formthe input to DC to AC converter 900 and the output of multiple seriesstrings 902.

Reference is now made to FIG. 10 which illustrates a method 10 accordingto an embodiment of the present invention. In step 11, DC voltage inputsare connected to inductive elements. In step 13, the inductive elementsare switched at a high frequency dependent on the inductance values sothat the inductive elements do not tend to “short ” the input DCvoltages. In step 15, a single output combines the DC inputs byconnecting across typically the highest input voltage and a reference orground common to both the DC inputs and the single output.

The definite articles “a”, “an” is used herein, such as “amulti-junction photovoltaic cell”, “a power combiner” or “a coil” havethe meaning of “one or more multi-junction photovoltaic cells”, “one ormore power combiners” or “one or more coils”.

Although selected embodiments of the present invention have been shownand described, it is to be understood the present invention is notlimited to the described embodiments. Instead, it is to be appreciatedthat changes may be made to these embodiments without departing from theprinciples and spirit of the invention, the scope of which is defined bythe claims and the equivalents thereof.

1-17. (canceled)
 18. A method, comprising: connecting at least two inputdirect current (DC) voltages to at least two primary inductors via atleast two primary switches and to at least two secondary inductors viaat least two secondary switches of a circuit, wherein the at least twoprimary inductors are coupled to the at least two secondary inductors,wherein a first DC voltage is connected to a first primary inductor viaa first primary switch and to a first secondary inductor via a firstsecondary switch, and a second DC voltage is connected to a secondprimary inductor via a second primary switch and to a second secondaryinductor via a second secondary switch, wherein at least one inputterminal is shared between the at least two input DC voltages; switchingthe at least two primary switches and the at least two secondaryswitches at different times; and outputting a combined voltage at anoutput of the circuit, the combined voltage being a sum of the at leasttwo input DC voltages.